1. Field of the Invention
The present invention relates to a liquid crystal optical element, and more particularly to a liquid crystal optical element using a chiral smectic liquid crystal having the spontaneous polarization and with the optical response to this action.
2. Related Background Art
Ferroelectric chiral smectic liquid crystals as the liquid crystal having the spontaneous polarization have been actively developed for the usages of a display element and a light bulb, because of their noted advantages of high speed responsibility and memory property. Apparatuses utilizing the above-mentioned advantages that have been put to practical use include a light shutter arrays, a high definition display unit with the simple matrix drive, and a light bulb for the high density recording in combination with a photoconductor.
Application is also expected to a motion picture display with the active matrix drive using thin film transistors (TFT), with this characteristics being shown in, for example, U.S. Pat. No. 4,840,462 or Proceeding of the SID, vol. 30/2, 1989 "Ferroelectric Liquid Crystal Video Display". For example, the drive method of this type of liquid crystal optical element is one in which the reset and write signals are inserted in time division into a horizontal period, as described in U.S. Pat. No. 4,840,462. Also, for example, the element for gradation display with the above drive method was fabricated in a process in which an electrode layer 12 is formed on a substrate 11 of glass, then an orientation layer 13 for orienting a chiral smectic liquid crystal 14 is provided, and then subjected to the rubbing, and the chiral smectic liquid crystal 14 is injected thereto, as shown in FIG. 7.
However, in driving the conventional liquid crystal optical element as above mentioned, the following problems generally arose. One of them was that if the DC voltage (DC component) was continuously applied to a chiral smectic liquid crystal for a long time, the response of liquid crystal might be neglected. This cause is considered due to the fact that the deflection of internal ions of liquid crystal is induced by the above DC component to form an electric field. On the contrary, a proposal has been made of cancelling the DC component with the auxiliary pulse. Further, it is considered that with the chiral smectic liquid crystal, internal ions deflected by the spontaneous polarization of molecules themselves will form an electric field. On the contrary, another proposal has been made in which the half tone can be stably formed, without giving the effects of spontaneous polarization to the next frame, by virtue of an element constitution of using a chiral smectic liquid crystal material having a time constant (CR) for relaxing the internal electric field induced by the above spontaneous polarization within a reset interval, for example, as disclosed in European Patent Gazette No. 448,032. However, there was a problem in making the domain gradation of chiral smectic liquid crystal, that at the same time when electric charges are moved between the electrode, the orientation layer and the chiral smectic liquid crystal, the domain gradually expands to exert an adverse influence particularly for realizing the high gradation, owing to the influence of charges moving through the interface of chiral smectic liquid crystal.
Note that the domain gradation is one in which the crest value or pulse width of applied voltage is set at a value corresponding to a gradation to make a display by varying the ratio between the white domain and the black domain within a pixel depending on the applied voltage, as described in U.S. Pat. No. 4,655,561, U.S. Pat. No. 4,712,877, U.S. Pat. No. 4,765,720, and U.S. Pat. No. 4,796,980, for example.